Donald Trump's warning that, as president, he may not support the automatic defense of a North Atlantic Treaty Organization ally if it is attacked, may well be evidence of the Republican nominee's isolationist views. But more likely, it reflects a belief that rules can be bent when they don't suit him. It has defined the Trump candidacy and it will define his presidency if he wins.

Trump told The New York Times that if Russia attacks the Baltic states, he would first see if these countries "have fulfilled their obligations to us" and then decide whether the U.S. should intervene. Technically, of course, nations' obligations under the North Atlantic Treaty are not to the U.S. but to all of their allies, but that small error may be forgiven because of the huge disparity in military spending between the U.S. and the other NATO states.

Trump's approach would mean Estonia would be the only Baltic nation to qualify for U.S. intervention in case of an attack: It complies with the treaty's requirement that countries spend 2 percent of their economic output on defense. Latvia and Lithuania have pledged to catch up in the next several years, but if they are attacked, say, next January, the day after President Trump's inauguration, Russian President Vladimir Putin can probably have them.

That's useful for him to know; even if he's not planning an attack, it becomes a tantalizing possibility. It's hard to see Latvia and Lithuania scrambling to draw up new budgets for 2017, especially since a NATO summit this month reaffirmed the alliance's commitment to defending them. Theoretically, since an international treaty is involved, any U.S. president has to continue the line President Barack Obama took at the summit, saying, "Here in Warsaw, we haven’t simply reaffirmed our enduring Article 5 obligations to our common security; we’re moving forward with the most significant reinforcement of our collective defense any time since the Cold War."

Trump made his candid remarks in the populist spirit of isolationism. "We're going to take care of this country first," Trump said, adding that he doesn't feel Americans have a right to "lecture" other countries, such as Turkey, now in the midst of a massive purge after a thwarted coup attempt, until the U.S. can present more of a shining example. (Actually, the U.S., as any NATO country, has the right to "lecture" fellow members about democracy -- and Obama did criticize Poland for its government's recent undemocratic leanings -- if only because the treaty says the signatories are states "founded on the principles of democracy, individual liberty and the rule of law.")

Trump may have forgotten that the U.S. is the only country for which Article 5 has ever been invoked -- after 9/11, when America's allies stood with it against the terrorist menace. But no matter how much of a political novice Trump is, he must understand that the U.S. cannot just isolate itself: It's been going in the opposite direction for too long, and it has accumulated obligations along the way.

Article 5 -- the whole reason for the military alliance's existence -- is worded rather carefully: Countries are obliged to take "such action as it deems necessary" to help any ally who has been attacked, but military action is not explicitly required. If previous U.S. governments saw this as a binding commitment to use military force and led others to see it that way, too -- that's why the Baltics joined NATO in the first place -- Trump is saying he's willing to reinterpret Article 5 in his favor, to bend what everyone considers to be a firm rule. In plain language, to renege.

That kind of dissembling shouldn't be a surprise to anyone. Trump's disregard for the facts, and normal political conventions, has been a surprisingly harmless staple during the presidential campaign, though even there it has led to some awkward moments.

Tony Schwarz, who ghostwrote Trump's bestselling book, "The Art of the Deal," has been telling anyone who would listen that it was probably filled with inaccuracies. Trump's lawyer recently sent a cease and desist letter to Schwarz, demanding the writer's advance and royalties back.

It's clear that Trump tolerates the same opportunistic attitudes in others. Melania Trump's plagiarized speech at the Republican convention didn't elicit an apology from Trump or Melania -- just from a hapless campaign staffer whose offer to resign was rejected.

His campaign managers, Paul Manafort and Rick Gates, have traveled in circles where connections matter more than rules. Trump's approach to business, politics and life in general is based on a belief in the survival of the fittest, which apparently includes cutting corners when the reward is greater than the risk. If elected, this modus operandi risks becoming official policy. The world would be likely to see a different America, whose word is no longer its bond; capable of a stab in the back or a self-serving feint.

I have often heard and read that Trump's danger as a potential president is exaggerated, that the U.S. system runs itself, that checks and balances compensate for leaders' character deficiencies. Any system, however, can be subverted and turned into a travesty of itself by using every possible loophole in agreements and conventions. I have little doubt that Trump can pull it off.

This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.